1. Technical Field
The present invention relates to a communication apparatus, a communication method, and a communication system capable of carrying out communication using communication parameters suitable to states of a transmission channel.
2. Background Art
When impulse noise or phase fluctuation occurs in a transmission channel such as communication using a power line or mobile wireless communication or when the amplitude or phase of a reception signal varies due to fading or the like, distortion may occur in the reception signal, thereby increasing a bit error of reception data.
As a method of solving such a problem, a method of inserting a pilot symbol into a reception signal is disclosed in Patent Document 1 (JP-A-2002-84332) and Patent Document 2 (JP-A-2006-352492), for example. The pilot symbol is a known signal. Generally, the plural pilot symbols are inserted at periodic interval. Accordingly, since a characteristic of a transmission channel can be newly estimated (that is, variation in a state of the transmission channel is detected) by monitoring the phase or amplitude of the reception signal in a block of the pilot symbols, an increase in the bit error can be avoid by equalizing the reception signal on the basis of the state of the newly estimated transmission channel.
FIG. 11 is a diagram illustrating an example of a frame format of a communication frame when the pilot symbol is inserted. As shown in FIG. 11, the communication frame includes a preamble PR, frame control data FC, an information symbol (payload) PLD, and a pilot symbol PLT. The preamble PR is data used for symbol synchronization, equalization coefficient calculation, and the like. The frame control data FC is control data used to control information on a transmission source address, a transmission destination address, a form (length of a frame, etc.) of the payload, etc. The payload includes the information symbol PLD and the pilot symbol PLT. The pilot symbol PLT having an M length is inserted into every information symbol PLD having an N length.
The pilot symbol PLT is the known data, as described above, and data which do not contribute to information transmission. Accordingly, when communication is carried out using frames having such a structure, transmission efficiency is reduced as many as the number of the pilot symbols PLT, compared to a case where the pilot symbol PLT is not inserted. In the example of FIG. 11, the transmission efficiency is reduced by the maximum M/(N+M).
When communication is carried out using the communication parameters suitable for states of a transmission channel, the states of the transmission channel are estimated by a channel estimation (hereinafter, also referred to as CE) and a new communication parameter is obtained on the basis of the estimated result. Subsequently, it is confirmed by a CE training process in which one of a current (in use) communication parameter and the newly obtained communication parameter is more suitable for the state of the transmission channel to determine the communication parameter to be used.
The CE training process is disclosed in Patent Document 3 (WO05/011226A), for example.